In recent years, a laser beam has been used for various applications, for example, for cutting or processing a metal, a light source of a photolithography device of a semiconductor manufacturing device, various measuring devices, and operating and treatment devices for surgery, ophthalmology, and dentistry.
When a solid-state laser (cited as a concept including semiconductor laser (or diode laser) in this specification) is used as a laser light source, the wavelength of a laser beam emitted from the solid-state laser is in the visible to infrared region. A method of directly generating an ultraviolet light has not been established. For example, the wavelength is too long to be suitable for use in an inspection device. A method of converting such long-wavelength light emitted from the solid-state laser to a short-wavelength deep ultraviolet light (for example, the eighth harmonic; a wavelength of 193 nm) using a nonlinear optical crystal has been developed and has been described in Japanese Patent Application Laid-Open (JP-A) No. 2001-353176 (Patent Document 1). As nonlinear optical crystals used for such an object, a BBO crystal, an LBO crystal, and a CLBO crystal have been known.
In such laser light source, typically, a laser beam generated from a DFB-LD is amplified using a plurality of optical fiber amplifiers (for example, EDFAs) and is then converted to the deep ultraviolet light by the wavelength conversion optical system.
Patent Document 1: JP-A No. 2001-353176
To maintain the intensity of the output laser beam at a target value, in such a laser light source, feedback control is typically performed. FIG. 7 illustrates the overview of such light source device. An output light from a DFB-LD1 comes into an optical fiber amplifier 3 and is then amplified. The light passes through a polarization state adjusting optical element 4 having a wavelength plate to come into a wavelength conversion optical system 5. The light is wavelength converted to a light having a target wavelength.
The polarization state adjusting optical element 4 adjusts the polarization state of the laser beam coming into the wavelength conversion optical system 5 in such a way that the wavelength conversion optical system 5 has the maximum conversion efficiency. For example, the polarization ellipticity is adjusted by a ¼ wavelength plate. The polarization direction is adjusted by a ½ wavelength plate.
A portion of the output light from the wavelength conversion optical system 5 (for example, the light which has a wavelength of 193 nm and is the eighth harmonic of the light having a wavelength of 1547 nm from the DFB-LD1) is reflected by a partially reflecting mirror 6 and is then taken out as a monitor light. An automatic output controller 7 manipulates an excitation light source (pump light source) 8 which supplies an input to the optical fiber amplifier 3 in such a way that the intensity of the monitor light is kept constant. It has been thought that the intensity of the output light from the wavelength conversion optical system 5 is maintained at a target value by the feedback control system.
Actually, however, a change in the output light from the wavelength conversion optical system 5 for an increase or a decrease in the pump light intensity sometimes greatly differs from that is assumed. This means that the feedback control sometimes does not function sufficiently. In extreme cases, when the output of the pump light is increased, the output of the wavelength conversion optical system 5 is lowered. This means that the feedback control causes divergence.
The present invention has been made in view of such circumstances and can provide an optical fiber amplifier in which even if the intensity of a pump light is changed, a change in the polarization state of an output light is small and the feedback control system can function sufficiently, and a light source device using the same, and an exposure device, an object inspection device, and a processing device using the light source device.